The invention relates to an ATM Network Element enabling automatic protection switching of a Transmission Convergencence Sublayer SubNetwork Connection (TCS SNC). The invention equally relates to a method for automatic protection switching of a Transmission Convergencence Sublayer SubNetwork Connection between a first ATM Network Element and a second ATM Network Element.
Transmission Convergencence Sublayer SubNetwork Connection are established between Transmission Convergence Sublayer entities of different ATM Network Elements of an ATM network, the Transmission Convergence Sublayer entities providing as part of the physical line interfaces of the ATM Network Elements access to physical transmission paths.
Protection schemes based on Synchronous Digital Networks (SDH) are well known in the art. They provide quick switch reaction to failures and short recovery times.
The use of the protection schemes based on the use of SDH and/or PDH transmission functions in an ATM Network, however, has the disadvantage of requiring additional transmission equipment in addition to the ATM Network Elements.
A proposal for a protection scheme for ATM transport networks based on the use of the ATM layer can be found, e.g., in U.S. Pat. No. 5,838,924. This protection scheme makes use of a virtual path group transport architecture comprising a working route, a protection route, and VPG bridge and selector functions at each end of the routes. Defect type indication is to be provided by a new segment level OAM cell. In case a defect is detected in the VPG assigned to the working route, the receiving node switches for reception to the protection route.
Protection schemes based on the use of the ATM layer, however, are not able to yield a performance, in terms of failure recovery time, comparable to that of SDH transport networks.
It is an object of the invention to provide a simple protection scheme which is suitable to work on ATM networks and is able to match switching requirements of current transmission networks (about 50 ms).
This object is reached by an ATM Network Element enabling automatic protection switching of a Transmission Convergence Sublayer SubNetwork Connection (TCS SNC) and comprising at least one working Transmission Convergence Sublayer entity as part of a first physical line interface and at least one protection Transmission Convergence Sublayer entity as part of a second physical line interface, both entities, respectively, being individually addressable within the ATM Network Element via an assigned UTOPIA address, the ATM Network Element being provided by the Transmission Convergence Sublayer with physical layer error messages indicating failures in the Transmission Convergence Sublayer SubNetwork Connection established via the working Transmission Convergence Sublayer entity, the ATM Network Element being configured to select for transmission of ATM-cells the UTOPIA address of the working Transmission Convergence Sublayer entity as long as no physical layer error message is received, and, after reception of a physical layer error message, the UTOPIA address of the protection Transmission Convergence Sublayer entity, and the ATM Network Element being configured to merge the data entering the ATM Network Element via both, the working Transmission Convergence Sublayer entity and the protection Transmission Convergence Sublayer entity.
The object is equally reached by a Method for automatic protection switching of a Transmission Convergencence Sublayer SubNetwork Connection (TCS SNC) between a first ATM Network Element and a second ATM Network Element in an ATM network, each ATM Network Element comprising at least one working Transmission Convergence Sublayer entity as part of a first physical line interface and at least one protection Transmission Convergence Sublayer entity as part of a second physical line interface, said working Transmission Convergence Sublayer entities of first and second ATM Network Element and said protection Transmission Convergence Sublayer entities of first and second ATM Network Element enabling independent Transmission Convergencence Sublayer SubNetwork Connections via different transmission paths, each of the Transmission Convergence Sublayer entities being addressable within the first and the second ATM Network Element respectively via a dedicated UTOPIA address, and each Transmission Convergence Sublayer entity providing physical layer error messages indicating failures in the Transmission Convergencence Sublayer SubNetwork Connection, the method comprising the steps of:
as long as no failure in the used Transmission Convergencence Sublayer SubNetwork Connection is indicated by a provided physical layer error message,
a) transmitting data from the first ATM Network Element employed as source point via the corresponding working Transmission Convergence Sublayer entity, selected by its assigned UTOPIA address; and
if a physical layer error message indicating a failure in the used Transmission Convergencence Sublayer SubNetwork Connection is provided by the working Transmission Convergence Sublayer entity of the second ATM Network Element:
b) transmitting a failure message to the first ATM Network Element;
c) changing the UTOPIA address used for addressing ATM cells to be transmitted to a Transmission Convergence Sublayer entity in the first ATM Network Element from the UTOPIA address assigned to the working Transmission Convergence Sublayer entity to the UTOPIA address assigned to the protecting Transmission Convergence Sublayer entity;
d) transmitting ATM cells from the first ATM Network Element via the corresponding protection Transmission Convergence Sublayer entity selected by its assigned UTOPIA address; and
regardless of whether the working or the protection Transmission Convergence Sublayer entity is used for transmission in the first ATM network element,
e) merging all data received via working Transmission Convergence Sublayer entity and protection Transmission Convergence Sublayer entity of the second ATM Network Element employed as sink.
The invention proceeds from the fact that in ATM networks, typically, ATM Network Elements use UTOPIA addresses to identify independent Physical layers and, hence Transmission Convergence Sublayer entities or Service Access Points which give service to the ATM layer. Moreover, the invention takes advantage of that the Transmission Convergence Sublayer, within the Physical layer of the ATM protocol stack, offers alarms and monitoring functions which are an extension of those of High Order Transmission Path of SDH and/or PDH transport networks. These alarm and monitoring functions have been standardized by ITU-T Recommendation I.610, which describes different operation and maintenance (OAM) flows (F1-F6). The physical layer contains the three lowest OAM levels, OAM flow F3 being allocated to the transmission path level.
The ATM Network Element and the method according to the invention make use of those functions to trigger protection scheme actions on the Transmission Convergence Sublayer of ATM Network Elements by performing switching of UTOPIA addresses within the ATM Network Element for transmission of ATM cells. Internal UTOPIA addresses are therefore handled to switch quickly from the working Transmission Convergence sublayer entity to the provided protection Transmission Convergence sublayer entity. This scheme allows to associate a protection physical line interface to a working physical line interface to perform Transmission Convergense Sublayer SubNetwork Connection protection with inherent or non-intrusive monitoring (TCS SNC/I or TCS SNC/N).
It is an advantage of the ATM Network Element and the method of the invention that they enable a protection scheme yielding a very short recovery time, since it is only necessary to change the UTOPIA address at the source point of the traffic in case of a reported failure. No operation is required at the ATM layer neither in the source nor the sink points.
The switching function is triggered by the detection of standard alarms, in particular standard alarms in the F3 OAM flow. No ad-hoc automatic protection switching protocol is required.
It is a further advantage that the protection mechanisms according to the invention is based on the use of Transmission Convergence Sublayer functions suitable to be implemented in ATM networks without the need to employ additional PDH or SDH transmission equipment.
The proposed protection scheme may be considered as an extension of high order path SubNetwork Connection protection and it is a simplified way to implement protection schemes in ATM networks.
The proposed merging at the sink point of an ATM Network Element is possible due to the fact that during normal operation no cell is sent to the ATM layer by the protection Transmission Convergence Sublayer SubNetwork Connection and that no cell is sent to the ATM layer by the working Transmission Convergence Sublayer SubNetwork Connection when the underlying physical line is in failure.
The proposed switching at the source point in case of received error messages and the merging at the sink point make the permanent transmission of data via two different TSC SNCs for an effective protection superfluous.
Advantageously, a dual ended switching is employed. Both directions of transmission are switched from working to protection Transmission Convergence Sublayer entities even after a solely unidirectional failure or signal degrade condition. The coordination of switches relies on the use of the OAM F3 flow and does not require any APS protocol.
Since Transmission Convergence Sublayer functions are implemented on all types of physical line interfaces of ATM Network Elements, TCS SNC protection scheme is applicable to all types of physical line interfaces, in particular SDH or PDH, including the cases where Inverse Multiplexing for ATM (IMA) is used over PDH links.
In a preferred embodiment the features of the TCS SNC scheme are extended to protect Virtual Path Connections which coincide with the span of the Transmission Convergence Sublayer. This is done by associating a protection Virtual Path Connection which terminates on the same access point as the TCS SNC to the working Transmission Convergence Sublayer. This implies that the TCS SNC protection switching scheme is able to protect Virtual Path Connections which are established between adjacent ATM Network Elements. In a ring topology, the protection virtual path contains intermediate ATM Network Elements. The working and protecting Virtual Path Connection must include the merging functionality on the sink termination points. The protection Virtual Path Connection is set up simultaneously with the working Virtual Path Connection by means of the management plane.
Equally, Virtual Circuit Connections may be protected.
Preferably, a revertive operation is chosen for the method according to the invention. This means that the traffic on the protection Transmission Convergence Sublayer entity shall be switched back to the working Transmission Convergence Sublayer entity when it has recovered from a fault or signal degrade condition. This characteristic is needed to be able to protect Virtual Path Connections which coincide with the span of the Transmission Convergence Sublayer (Virtual Path Connection within adjacent ATM Network Element).
In order to enable a revertive operation, the possibility of setting a Wait To Restore condition is provided. In this condition, the working Transmission Convergence SubNetwork Connection meets the restoration threshold after a fault or signal degrade condition. It therefore indicates that the transport of working traffic is ready to be reverted to the working Transmission Convergence SubNetwork Connection from the protection Transmission Convergence SubNetwork Connection.
The ATM Network Element and the method of the invention may associate heterogeneous physical line interfaces to provide protection on point to point connections and loop network scenarios. The protection scheme is applicable to more complicated network scenarios such as rings due to the merging at the sink point.
The ATM Network Element and the method according to the invention are especially intended for the use in the scope of a Radio Access Network of the WCDMA (Wideband Code Division Multiple Access) Third Generation Networks.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.